Your mode door is controlled by vacuum check the vacuum hose on the left side of the intake manifold. these some times fall off leaving your mode door with no vacuum and the default position of the mod door is defrost.

You have a vacuum leak.Defrost is the default postion(no vacuum).
See if you can locate the small black vacuum line that goes inside the vehicle on the passenger side near the heater core. The trace it back,should see it going to a canister then to the intake manifold.Make sure it is not disconnected or broken somewhere.

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This is caused by a low engine vacuum, the engine vacuum is low when you accelerate, the HVAC system loses vacuum to the mode control vacuum switch and defaults to the defrost ducts for safety. You likely have a leaking mode control vacuum switch in the control panel of the heater AC system (HVAC).

Sounds like the switch behind the selector is eithe broken, or stuck in that posiition (or internally) is not moving. I suggest pulling the heater/ac selector unit out and checking the levers from the back side.
Please let me know if you need further assistance.
carsandcomps,
Guru, Premium Expert @ Fixya.com

In every car, there are fins or flaps inside the dash that rotate that distributes the air flow from the blower to the desired opening, ie defrost, floor etc... This flap is controlled by a linkage which is connected to the selector switch. It may be the linkage is no longer connected, or the fins inside the dash are stuck. If the selector switch moves freely with little or no resistence its possible the linkage is no longer connected, otherwise the flaps need to be physically looked at.

the turn signal/multifunction switch is defective, the other problem is a vacuum loss at the AC mode control switch or the vacuum switch itself (part of the AC control in dash), check the vacuum resovior under the hood gfor leaks and make sure the vacuum reaches the switch in the dash.

U have either lost the vacuum to the mode selector switch (in the dash control) or the vacuum mode selector switch is leaking in the ac/heater control in the dash, u can usually hear a hiss if it is leaking. the vacuum source is the engine, a small hose runs through the firewall on the passenger side to the switch from the eng vacuum source.

with the truck turned off pull the gear selector down to 1, tilt steering wheel as far down as it will go. now on the dash pull the trim ring 'bezel"
this is the piece that goes aroung the headlight switch over the gauges around the radio and the climat contols. once that is off the switch has to clips on on each side pull back both at once or one at a time the module will come right out